The RPC module type is the standard module signature that the various specialization modules must conform to.
The implementation is dependent on the module, and represents the 'result' of the entire module. For example, in the Server module, the `implementation` is the server function, with type
Rpc.call -> Rpc.response.
For the Client module, the individual declarations are used to perform the RPCs, and the 'implementation' type is simply unit.
To actually construct the implementation, an interface description must be provided
'a res is the result type of declaring a function. For example, the Client module, given an (int -> int -> int) fn, will return a function of type 'a - in this case, (int -> int -> int)
This is for inserting a type in between the function application and its result. For example, this could be an Lwt.t, meaning that the result of a function application is a thread
This infix operator is for constructing function types
Require a unit argument in OCaml without sending a parameter. Useful for methods that take no arguments.
declare name description typ is how an RPC is declared to the module implementing the functionality. The return type is dependent upon the module being used